1. Field of the Invention
The present invention relates to an image coder and an image decoder that are capable of efficiently reducing power consumption during compression and decompression of image data, including video data (motion picture image data) and digital still image data.
2. Description of the Related Art
A technique for performing power-saving operations for image compression in encoding video data is known. For example, Japanese patent application 2001-238189 discloses a technique for selecting appropriate resolution and/or frame rate of image data in accordance with the remaining charge of the battery, and for varying the clock rate and the power supply voltage of the associated circuit(s) based on the selected parameters, in order to reduce power consumption.
On the other hand, a new standard for image compression and decompression, JPEG 2000, is being established.
To reduce power consumption in compression of image data using algorithms of JPEG 2000, not only the resolution and the frame rate of the image data, but also other adjustable factors, such as compression rate control, tiling mode, the number of taps of the wavelet filter, and the hierarchical level of the wavelet transform, have to be taken into account. The technique disclosed in Japanese patent application 2001-238189 cannot be applied to the reduction of power consumption in image compression and decompression based on the JPEG 2000 standard because that publication does not disclose any control for the JPEG 2000 factors.
In addition, Japanese patent application 2001-238189 is directed only to power-saving control in encoding image data, and it does not address power-saving control for decompression when decoding the encoded data stream.
With the conventional techniques, all the encoded data have to be decoded, and therefore, it is difficult for the conventional techniques to reduce power consumption by reducing the amount of data to be processed.